The overall goal of this project is to further an understanding of the genetic, environmental, and cultural influences that are involved in the etiology of human breast cancer. The specific aim is to test for genetic linkage between a large array of discrete, polymorphic genetic markers and the gene(s) for breast cancer in family data. The ultimate goal is to localize a gene or genes that predispose women in high-risk families to breast cancer. A sample of women with a strong family history of breast cancer who participated in the Breast Cancer Detection Demonstration Project (BCDDP) will be contacted and pedigree, vital status, health history, and epidemiological data will be collected from them and their family members. Fifteen to twenty families whose pedigree structure appears to be the most informative for use in linkage analysis studies will be selected. Blood will be collected from family members and analyzed for the presence of a number of genetic markers, including blood group antigens, red blood cell enzymes, plasma proteins, and restriction fragment length polymorphisms (RFLPs). Marker data results will then be used to perform computer generated linkage analysis.